In order to measure the conductivity of the combustion gas present in a cylinder of a diesel engine, the glow plug of the cylinder can be used as a sensor and a respective ion measurement can take place via said glow plug. The glow plug of the respective cylinder can be connected in series with a measurement or reference resistor and can in particular be conductively connected to the interior wall of the cylinder. An electrical voltage is applied to the glow plug for the determination of the conductivity of the combustion gas. If the conductivity of the combustion gas should be determined on the basis of the positively charged particles contained in the combustion gas, a negative voltage can be applied to the glow plug, for example, during part of the compression stroke and during part of the power stroke of the piston of the relative cylinder. The conductivity of the combustion gas between the glow plug and the interior wall of the cylinder changes due to the positively charged particles arising during the combustion process, whereby the voltage changes which drops at the measurement or reference resistor and which is measured and output as a measured signal.
In all cases in which ion measurements are carried out using a voltage which is different from the voltages present in the control unit of the relevant vehicle, a second power supply is required. For different technical reasons, a negative voltage is applied to the glow plug in many cases to carry out a respective ion measurement. Negative potential is applied to the glow plug terminal and positive potential to the plug end at the ground side. The current flow between the plug tip and the cylinder head determines the shape and amplitude of the ion measurement signal during the combustion process.
Up to now, a second power supply always had to be integrated into the control device or into the glow plug controller in order to provide the negative voltage. For this purpose, for example, a DC-DC converter or a charge pump can be used. Such solutions are, however, relatively complex and expensive.